Sony STR-SE501 Loud Hum in all channels

[kingsxwc]

I've got a receiver that has developed what sounds like a loud 60 hz hum on all channels except the subwoofer. When the volume is turned all the way down, it stops but as soon as you turn the volume to "1" it comes on. You can turn the volume up all the way and the volume of the hum doesn't change. Mute will also stop the hum. When the A/C unit in the house cuts on, there is a momentary drop in the household voltage (the lights dim a bit) and if the receiver is on, the home gets very loud until the initial starting surge from the A/C is gone. I've downloaded the service manual and will start tracing signals shortly, but I was hoping somebody may have seen this before and may have an idea what might be causing this. I'm an electronics technician but I don't work on home audio equipment so feel free to get as technical as you want. Thanks.

[torbjorn]

Most likely, the problem is due to a bad filter capacitor in the power supply or an open-circuited diode in some bridge rectifier.

Check out the ripple voltage on the supply rails by oscilloscope and see if there is remarkably much ripple on any of them. Or, if the ripple is 50/60 Hz instead of 100/120 Hz as it should be after a full-wave rectifier where everything is in order.

Another possibility is that when positive and negative voltages (for instance, ± 12 V) are needed, those are supplied by a bridge rectifier which is fed from a transformer winding with a grounded center tap. If now there is an open circuit between any end of the winding and the rectifier, then both voltages will still be there but half-wave rectified instead of full-wave. It is very common to have a fuse or fusible resistor in series with each end of the transformer winding in such cases.

[kingsxwc]

Thanks for the quick reply. I'll definitely check the pwr supply.